The present invention relates to a high solids content, aqueous-based, self-contained printing ink (SC ink) and method of making, and further, to a carbonless copy paper sheet that has the ink coated thereon.
It is well known to those skilled in the art that pressure-sensitive recording paper, better known as carbonless copy paper, is useful in a variety of systems. For example, computer print outs and other pressure marking applications where it is advantageous to eliminate the necessity of typewriter ribbon or independent ink supply have particular use for such carbonless copy paper. It is also well known that such carbonless copy paper systems are produced by forming a layer of pressure-rupturable microcapsules containing a solution of colorless dyestuff precursor on the back side of the front sheet of paper of a carbonless copy paper set. This coated back side is known as the CB coating. In order to develop an image or copy, the CB coating must be mated with a paper containing a coating of a suitable color developer, also known as dyestuff acceptor, on its front. This coated front color developer coating is called the CF coating. The color developer is a material, usually acidic, capable of forming the color of the dyestuff by reaction with the dyestuff precursor.
Marking of the pressure-sensitive recording papers is effected by rupturing the capsules in the CB coating by means of pressure to cause the dye precursor solution to be exuded onto the front of the mated sheet below it. The colorless or slightly colored dyestuff, or dyestuff precursor, then reacts with the color developer in the areas in which the dye containing microcapsules were ruptured, thereby affecting the colored marking. Such mechanism for the technique of producing pressure-sensitive recording papers is well known.
Further developments in this art have shown that microcapsules containing the dyestuff precursor and the unencapsulated color developer dispersion may be coated onto paper or other substrate in a single coating application. The production of self-contained carbonless copy paper having a color developer, a dyestuff precursor and a common solvent for each maintained in isolation on one surface of paper base is known. Examples of such self-contained carbonless copy sheets are described in U.S. Pat. Nos. 3,663,256, 3,672,935, 3,732,120, 3,732,141. Typically, in these patents one or both of the dyestuff precursor and the color developer are encapsulated and applied to the substrate as single or separate coatings. These coatings used an aqueous-based vehicle over the entire surface of the substrate.
The processes described in these patents have the disadvantage that, following application of the aqueous-based coating composition, a considerable amount of water must be evaporated, thus requiring significant expenditures of energy. Additionally, the need for drying such an aqueous-based coating composition requires the use of a complex and expensive apparatus. More particularly, when removing the water, numerous controls are required for the coating and drying apparatus to prevent sheet distortion, curl or cockle. Special grades of paper are also required to avoid excessive penetration and web breaks on the coating apparatus. The result is a coating and drying apparatus and accompanying facilities that are expensive to build and operate. The centralization of production, due to high capitalization costs, also produces cost inefficiencies in the form of high scrap levels and transportation costs. All of these factors add to the cost of the resulting self-contained sheet.
It is also known that high solids content coatings can be achieved by utilizing a liquid solvent that is capable of dissolving the color-forming reactants but not interfere with the color-forming reaction in a deleterious manner. As taught by U.S. Pat. No. 3,663,256, if a given solvent interferes with the color-forming reaction or diminishes color intensity of the marking, the solvent should be sufficiently volatile to assure its removal from the reaction site after it has brought the color-forming reactants into intimate contact. However, the consistency of these solid solutions and the necessarily volatile solvents used therein give rise to printing problems and cause swelling of rubber plates and rolls. Furthermore, they have high energy requirements and potential environmental contamination problems from the volatile solvents.
Accordingly, in U.S. Pat. No. 4,337,968, it is suggested that a non-volatile organic solvent be used as a partial substitute for the water vehicle. It is stated therein that in combination with an acidic color developer such as phenolic resin, a fatty alcohol, and an amorphous lipophilic silica, the non-volatile solvent improves the characteristics of the coated film because there will be a residue of solvent in the solidified gel structure of the spongy color developer film produced thereby. This high boiling point solvent, as retained in the deposited film, tends to act as a plasticizer and give flexibility to the deposited film, preventing excess dusting during subsequent printing and handling. However, addition of such non-volatile solvent as a water replacement has a detrimental affect on the resultant image intensity of the dried coating. The lowered image intensity in the high solids content, self-contained coating is believed to be attributed to the dilution of the color-forming reactants caused by the addition of the non-volatile solvent.
Also known in the art are the use of binders in order to improve both the rheological properties and the substrate adhesion properties of encapsulated coatings. The rheological properties are extremely important when the coatings are applied to the paper substrate by rotogravure of flexoprinting, as shown in U.S. Pat. Nos. 3,016,308 or 3,914,511. In particular, it is known that the percent solids for an aqueous flexographic self-contained coating must be well below 50% (i.e., 38-42%) or it will otherwise be simply too viscous for application. Binders aid in improving printability of high solids content coatings and further, provide high internal strength within the dried coating. However, binders also reduce image intensity by forming a film between the colorless dyestuff precursor and the acid color developer. Furthermore, the interaction between the non-volatile solvent and the binder creates an inhibitor for the color-forming reactants and thus, precludes complete reaction between the reactants.
Accordingly, there remains a need in this art for high solids content, aqueous-based, self-contained printing ink which has an improved image intensity when printed onto a substrate and has suitable rheological properties for printing carbonless copy paper.